Combine cornstarch and thyme in a bowl. Strain gravy through a large colander, pressing vegetables with a wooden spoon; discard solids. Strain liquid through a fine sieve, and whisk into cornstarch-thyme mixture. Return gravy mixture to saucepan over high heat, and bring to a boil. Reduce heat to medium low, and simmer, whisking constantly, until thickened and starch flavor is cooked out, 2 to 3 minutes. Season with 1/2 teaspoon salt. Serve warm.